This invention relates generally to the fabrication of elongated tubular articles and, more particularly, to an improved method and apparatus for the fabrication of cartridge cases or the like.
Ammunition cartridge cases and similar cup-shape workpieces have previously been made in many cases by forming a recess in a solid, square, or cylindrical blank in a forging press and only thereafter shaping the blank in a forging machine to stretch it and give it final form. Two separate machines are required in such an operation and the workpiece must be introduced into and taken out from one machine and then reintroduced into the next machine. Obviously, such operations are time consuming and expensive. Presses having several tools are known in which essentially the same operations are performed in a single machine, however, the workpiece must also be brought manually from one tool to the other in such a case.
Typical of the prior art (illustrating the use of separate tooling and/or forging presses for fabricating cartridge cases or the like) are: Kaul, U.S. Pat. No. 2,891,298 which illustrates the steps of forwardly extruding and drawing the walls of a rocket shell in the process of fabricating the shell utilizing separate dies; Parre et al, U.S. Pat. No. 2,736,085, illustrating the use of two dies and punches to fabricate a cartridge case from bar stock, with the second die and punch forwardly extruding the wall of the case following an initial press of the stock against a bottom plug; and Hilton et al, U.S. Pat. No. 3,498,221, which illustrates the use of separate tooling and presses for making cartridge cases by first backwardly extruding a solid, cylindrical blank, followed by a drawing operation to thin and elongate the walls of the extruded blank.
It is known to fabricate various tubular articles utilizing a single press and single set of tooling and Kreidler, U.S. Pat. No. 2,893,553, is illustrative of a method and apparatus for fabricating tubular metal articles open at both ends from an initially hollow stamping. Kreidler illustrates a single operational sequence wherein the hollow stamping is placed within a die cavity which is initially closed at one end by a cushion, and a separately translatable punch and mandrel first cause the back extrusion of the workpiece to form an integral head thereon and thereafter the cushion moves away from the end of the die to permit the forward extrusion of an elongated tubular portion.
It is known in the art to fabricate cartridge cases in a single sequence of operations utilizing a single set of tooling as is taught by Talbot-Crosbie et al., U.S. Pat. No. 2,140,775. Talbot-Crosbie et al., illustrates the use of a punch and mandrel designed to forwardly extrude a billet through a die in a process of fabricating a shell case, with the punch also functioning as a blank holder in cooperation with the die. A bottom cushion is used for an initial forging action, with the cushion thereafter yielding to permit extrusion of the shell. The operation is continuous and utilizes the same tooling for all forming stages. However, in certain applications, it is extremely important to precisely control the interior shape and dimensions of the cartridge cases and techniques such as taught by Talbot-Crosbie et al. do not admit of such precise control. In a process such as is taught by Talbot-Crosbie et al., the material of the workpiece will be extruded ahead of the mandrel and the extrusion speed will increase as the corner of the mandrel approaches and enters the die orifice causing the bottom of the shell casing being formed to actually move away from the head of the mandrel. The duration of this phase depends on whether the corner between the front face of the mandrel and the side of the mandrel is sharp, radiused, chamferred or tapered; being a minimum for a sharp corner. Obviously, with large tolerances on the dimensions of the interior shape of the part being produced, the method and apparatus of Talbot-Crosbie et al. may produce acceptable parts, but such large tolerances are not usually found in present-day cartridge cases.
Techniques for controlling the interior shape of tubular articles such as cartridge cases are known but generally involve the use of extremely complex and expensive equipment. By way of example, Frothingham U.S. pat. No. 2,368,980 illustrates an elaborate mechanism and process capable of maintaining the position of a mandrel fixed relative to a die, fixed relative to the closed end of the part, or variable and controlled relative to the part. Frothingham apparently has the potential to control the internal dimensions and contour of the part, but at the sacrifice of cost and simplicity.
In addition to the operational and tooling constraints imposed by the necessity of maintaining close dimensional tolerances on the interior of the case being produced, an additional constraint is imposed by the inability of the material of the workpiece to undergo more than a predetermined decrease in cross-sectional area or in wall thickness in a single forging operation. Such a predetermined reduction is a function of the ductility of the workpiece's material.
Shortly before the outbreak of World War II, it was discovered that ordinary steels increase enormously in ductility when exposed to hydrostatic pressures in the range between 300,000 and 450,000 psi. This effect, as it applies to metal deformation generally, was first systematically studied by P. W. Bridgman in the early 1940's and his studies were compiled into a book entitled "Studies in Large Plastic Flow and Fracture", McGraw-Hill Book Company, Inc. 1952. Bridgman conducted experiments in drawing and extruding wire through a die in a high pressure liquid environment and found very significant increases in the degree to which the cross-sectional area of the work material could be reduced in a single pass. However, Bridgeman stated that, if one attempts to extrude material (without the aid of the hydrostatic pressure from the high pressure liquid filling the chamber) by pushing directly on the material as with a piston, it will be found that the pressure will not be transmitted to the extruding orifice because of lateral friction on the sides of the chamber, so that pressures high enough to split the extruding chamber will be reached before extruding begins.
Since the Bridgman work, many other investigators have duplicated, continued, and improved upon the techniques for high pressure fluid extrusion. Nevertheless, most of the work that has been done has been limited to unusual materials of limited interest in everyday applications. In addition, the investigation of processing under pressure has been almost wholly concerned with fluid extrusion of rods and tubes.
In 1967, in a paper entitled "Controlled Ductility Forming" presented before the International Conference on Manufacturing Technology at the University of Michigan in September, 1967, by F. J. Fuchs, Jr., there was described a high pressure shell drawing technique making use of the combined action of both high pressure fluid on the draw punch and shell blank to effect the desired metal deformation. "Controlled ductility" forming as used by Fuchs is stated to encompass all forming processes which make use of the effect of high pressure on ductility. Fuchs goes on in the same paper to describe an extension of the deep drawing controlled ductility forming technique in the absence of a fluid medium. The basic tooling envisioned by Fuchs comprised a die plate and a punch plate which were milled out in such a way as to provide guide grooves and a back support for a set of four or more edge pressure members. These members create an iris configuration which can open or close without forming gaps at the corners of the work blank. These edge punches are driven inward by a set of four high capacity rams, and the cooperation of the edge punches and the die and punch plates pressing against the work blank can raise the state of stress in the blank to very high and controllable triaxial compression. According to Fuchs, the ductility of the work blank can be just as effectively controlled as was the case with high pressure fluid.
Notwithstanding the teachings of Fuchs, that the use of a high pressure fluid medium could be dispensed with in increasing the ductility of the workpiece, it must nevertheless be appreciated that Fuchs' system is exceedingly complex, expensive and difficult to control in that he utilizes a minimum of six distinct and separately controlled pressure applying members to effect the desired state of stress in the work blank.
In view of the foregoing, it is an object of the present invention to provide an improved method and apparatus for the fabrication of tubular metal articles, the internal dimensions of which are accurately controlled.
Another object of the instant invention is to provide an improved method and apparatus for the forging of cartridge cases or the like whilst providing precise control of the internal dimensions of such cartridge cases and making use of the principle of increased ductility of the workpiece under stress.
Yet another object of the present invention is to provide a method and apparatus for the forging of tubular metal articles such as ammunition shells or the like more quickly and more efficiently than has heretofore been effected.
It is a further object of the subject invention to provide an improved method and apparatus for the fabrication of metal cartridge cases or the like having precisely controlled internal dimensions utilizing a single set of tooling with the work blank positioned at a single work station.
It is still a further object of the present invention to provide an improved method and apparatus for the fabrication of metal cartridge cases or the like wherein a work blank is fabricated by a single set of tooling during a single continuous operating cycle, which is effective to combine several, individual steps of a multiple step process into a single continuous operation.
It is a still further object of the instant invention to provide a simpler and more efficient method and apparatus for fabricating metal cartridge cases, wherein a work blank is positioned at a single work station, subjected to sufficient stress to increase its ductility, and subsequently forged into a cartridge case during a single operating sequence with a single pass of a tool or forming member.